The Gurus never acknowledged that devis existed (which can be seen as demi-Gods or lesser deities) nor did avatars, deities.
Yes the Guru's absolutely beleived that the devi devte exist. It's very apparent from Gurbani.
Even many Hindus openly say they don't see them as tangible actual entities that existed, but rather they see them as a symbol that helps them visualize certain aspects of the Creator.
Many Hindus openly say that the Sikh Panth is a part of the wider Hindu Panth. So what?
The references to these in SGGSJ are in a metaphorical / allegorical sense because the larger audience at the time it was written knew what these symbols stood for and so they would recognize the symbolism in Gurbani.
Not all of the references are. Some may be but it is absurd to say that all of them are.